264 MISS MARIETTA PALLIS ON THE 
(2) By the fact that each size of reed forms an entire Plav, or an entire 
portion of a Plav. Were the different-sized reeds different varieties, they 
would certainly sometimes mingle, just as happens in the case of accom- 
panying plants which, as a rule, form separate societies. Further, the 
submerged layer of a Plav formed of thickets of different-sized reed can be 
separated into pieces of Plav formed entirely of one size of reed by vertical 
planes only: the different sizes of reed are never superimposed upon опе 
another. Were the different sizes different varieties, they would certainly 
sometimes form mixed strata, or horizontal strata superimposed upon each 
other, just as the swamp-plants Typha angustifolia, Linn., and T. angustata, 
Bory & Chaub., form horizontal strata above Phragmites communis, Trin., 
and Phragmites communis, Trin., В. flavescens, Gren. & Godr., respectively, 
and thus sometimes together build up one fen in the Norfolk Broads, or one 
Plav in the delta of the Danube (see Р]. 24, section П.). 
Since reed of any size may in the delta of the Danube form an entire 
Plav, or portion of a Play, from the base upwards, all these size-classes of 
reed, including slender reed, must originally have been swamp plants, 
although only one of them, namely stout reed, is to be seen in the swamps. 
The explanation of these apparently contradictory facts is, I believe, that a 
gradual change takes place in the reed whereby slender shoots are produced 
in place of the original stout ones, and that this change in the shoots. 
takes place only after the stout reed, the reed of the swamps, has become 
detached and reached the Plav stage. Thus stout and slender reed do. 
not represent different varieties. They are in fact the lower and higher 
branches of one vast branch-system, which do not co-exist but succeed each 
other. Stout and slender reed is thus made up of shoots, or minor 
individuals, of different age in generation. The production of increasingly 
slender shoots, or minor individuals, by the plant, the major individual, 
therefore indieates a gradual progressive morphological change in its con- 
stitution—a change which culminates in death, since the shoot-output begins 
to decline after the reed has reached the stage at which it produces the 
shortest slender shoots (see pp. 267 & 268). 
To what cause is the change in the reed-shoot attributable ? It may 
be either external to the reed, for example (1) poisoning by its decayed 
dead remains, (2) overcrowding of the rhizomes or the aerial portions of 
the reed, (3) change in the physical environment, that is to say, и change т 
the relation of the surface of the Plav to that of the water, and (4) inter- 
competition with the accompanying plants; or the cause may be inherent to 
the reed. It is to the last cause that the change in the reed is, I think, to 
be attributed, viz., to senile degeneration, the inherent progressive change 
that culminates in death. 
I believe this to be the explanation, first, because (a) slender reed is 
apparently quite as healthy as stout reed. There is no change in the 
